Digital computer systems have a history of continually increasing the speed of the processors used in the system. As computer systems have migrated towards multiprocessor systems, sharing information between processors and memory systems has also generated a requirement for increased speed for the off-chip communication networks. Designers usually have more control over on-chip communication paths than for off-chip communication paths. Off-chip communication paths are longer, have higher noise, impedance mismatches, and have more discontinuities than on-chip communication paths. Since off-chip communication paths are of lower impedance, they require more current and thus more power to drive.
When using inter-chip high-speed signaling, noise and coupling between signal lines (cross talk) affects signal quality. One way to alleviate the detrimental effects of noise and coupling is through the use of differential signaling. Differential signaling comprises sending a signal and its compliment to a differential receiver. In this manner, noise and coupling affect both the signal and the compliment equally. The differential receiver only senses the difference between the signal and its compliment as the noise and coupling represent common mode signals. Therefore, differential signaling is resistant to the effects that noise and cross talk have on signal quality. On the negative side, differential signaling increases pin count by a factor of two for each data line. Additionally, an empty wiring channel is usually added between each differential channel which further adds to the wiring inefficiency.
The logic levels of driver side signals are determined by the positive and ground voltage potentials of the driver power supply. If the driver power supply has voltage variations that are unregulated, then the logic one and logic zero levels of the driver side signals will undergo similar variations. If the receiver is substantially remote from the driver such that its power supply voltage may undergo different variations from the driver side power supply, then additional variations will be added to any signal received in a receiver side terminator (e.g., Thevenin's network). A Thevenin's network is the equivalent circuit of a network between two terminals consisting of the open circuit Thevenin's voltage, measured at across the two terminals, in series with the Thevenin's impedance. The Thevenin's impedance is determined as the impedance between the two terminals with all voltage sources replaced with short circuits and all current sources replaced with open circuits. These power supply variations will reduce noise margins if the reference has variations different from those on the received signals caused by the driver and receiver side power supply variations. Also cross-talk between adjacent channels may lead to reduce “eye” patterns defining detection margins for signal transitions.
There is, therefore, a need for a signaling scheme that generates a reference wherein receiver side and driver side power supply noise is common mode, wiring channels are reduced, and differential receivers may be employed to reject common mode noise from power supplies and from cross-talk coupling between adjacent data channels.